Fiber-reinforced composite materials, which are made up of reinforcing fibers and matrix resins, are lightweight and have excellent mechanical properties. As such, these composite materials have been widely used in a variety of structural and non-structural applications such as aerospace, automotive, infra-structure repair, marine, military, and sporting goods or other consumer products that must have sufficient toughness and impact resistance to endure many years of harsh service.
Epoxy resins, and to a lesser extent, unsaturated polyester resins, vinyl ester resins, phenol resins, and polyimide resins, have been employed as matrix resins in fiber-reinforced composite materials. The use of composite materials having polyimide resin matrices is increasing, however, where these materials are now recognized as preferred structural materials in aerospace applications, because of their lightweight and load-bearing characteristics and their oxidative stability at elevated temperatures.
Various methods or techniques such as prepreg, hand lay-up, filament winding, pull-trusion, resin transfer molding (RTM) and resin infusion (RI), have been used to produce fiber-reinforced composite materials.
Current technologies for making prepreg and composites from polyimides utilize solutions from the poly(amide) acids of these resins. Poly(amide) acid solutions are processed into prepreg with various reinforcing fibers. These poly(amide) acid solutions are of low solids contents and high viscosity. Therefore, the processing of these types of solutions requires overcoming significant problems such as solvent management and good fiber wet out from the high viscosity solutions. The resultant prepreg typically requires residual solvent contents of 20 to 25% by weight (approximately 2-3% water from thermal imidization reaction) for adequate tack and drape. This residual solvent must then be removed during the composite cure cycle. This material is hand-laid into composites which makes working with this type of material very labor intensive and costly.
One example of a prior art prepreg solution that when processed into prepreg with suitable reinforcing fibers yielded a high temperature resistant composite, was first marketed in the 1960s under the trade designation SKYBOND by Monsanto Corporation, 800 N. Lindbergh Blvd., St. Louis, Mo. 63167. The SKYBOND prepreg solution was prepared by prereacting 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) with ethanol using NMP as the solvent. Then, either m-phenylenediamine (MPD) or 4,4′-methylenedianiline (MDA) was added to the solution. No endcapping agent was employed. It was (and continues to be) difficult to fabricate low void composites using this type of chemistry. One of the reasons offered for this difficulty has been that during the cure process branching can occur by the reaction of amine end-groups with the bridging carbonyl group of the BTDA moiety leading to branching and intractability of the matrix resin.
The processability (and thus end-use applications) of these polyimides has been improved by lowering molecular weight and by using reactive endcapping agents to produce thermosetting polyimides.
Originally developed in the 1970s for military aircraft applications, these thermosetting polyimides are produced by dissolving an aromatic diamine, a dialkyl ester of tetracarboxylic acid and a monofunctional nadic ester endcapping agent in a solvent (e.g., alcohol).
The best known of these early thermosetting polyimides is PMR-15, which is shorthand for in-situ polymerization of monomer reactants. These thermosetting polyimides (i.e., benzophenone dianhydride//methylenedianiline//nadic anhydride) were originally developed at NASA Lewis for military aircraft engines and airframes, undergo crosslinking at 250° C. to 300° C. (482° F. to 572° F.), and demonstrate good thermal and mechanical properties. PMR-15 thermosetting polyimides, however, contain the hazardous compound methylenedianiline (MDA), which raises health and safety concerns.
PMR-15 cures with a condensation reaction, which means that water and alcohol are evolved and given off from the polymer chain during cure. PMR-15 does not give a clean two-stage cure. In particular, PMR-15 crosslinks while volatiles are being removed and decomposes during cure to evolve cyclopentadiene. Moreover, the volume of solvent removed such as water, methanol and cyclopentadiene, produces voids in the final composite in an amount equal to greater than 2 percent (%) of the volume of the composite.
A direct PMR-15 replacement with very similar chemistry but using less toxic monomers is the nadic end-capped polyimides RP-46. These thermosetting polyimides (i.e., benzophenone dianhydride//3,4-oxydianiline//nadic anhydride), like PMR-15, cure with a condensation reaction. The two-stage cure is not a clean two-stage cure. Moreover, the resulting composite contains voids in an amount of greater than 2% of the volume of the composite.
Other non-MDA containing polyimides include PETI (i.e., phenylethynyl terminated imide) resins, which represent a different polyimide chemistry. These resins were developed to address the need for structural matrix resins that could provide higher temperature performance and longer-term thermoxidative stability. While more expensive than PMR-type materials, these high temperature resins, which cure in an addition reaction without volatile evolution, can be used to produce substantially void-free composites. Unfortunately, these resins require extremely high cure temperatures of around 375° C./707° F., which are very close to the degradation temperature of these resins. In addition, such high cure temperatures may necessitate specialized autoclave or press equipment for some manufacturers.
A need therefore exists for polyamic acid and polyimide oligomers for use in making adhesives and prepreg, where the polyimide oligomers provide a clean two-stage cure at temperatures of less than or equal to about 370° C.
It is therefore an object of the present invention to provide such polyamic acid and polyimide oligomers.
It is another object to provide solutions suitable for use as prepreg solutions or as high temperature adhesives, which contain one or more of the above-described oligomers and an organic solvent.
It is yet another object of the present invention to provide high temperature, low void volume, fiber-reinforced composites prepared from prepregs made using the inventive prepreg solutions.